The claim of a returning self-payment matched payment_hash and
payment_secret but not the HTLC amount. Answering the hook with
resolve settles the HTLC at once, so lightningd's own
final_incorrect_htlc_amount check is skipped. The last-hop peer
relays our onion (and thus the secret) intact but chooses the offered
amount, so it could settle a reduced HTLC, learn the preimage, and
claim the full amount upstream.
Record the intended amount at Claimer::generate() time and resolve
only an exact match; a mismatch is left to normal handling, which
fails the HTLC for lack of an invoice.
The same issue was recently fixed in sling (daywalker90/sling@835f36e8).
Fixes#322.
